New config files for embedded boards.
Build with ../nanobsd.sh -c rpi.cfg, for example.
This can be done as a normal user.

This is a work in progress. It relies on the new nopriv
build stuff committed to nanobsd, but isn't complete yet.
Currently, one must copy files into the DOS partition
in the image. Also, ownership isn't preserved because
this doesn't use the new mtree-dedup.awk yet, but rather
some crazy mtree stuff. The image building bits will
move up into nanobsd when they are ready.

Also includes very preliminary support for building qemu
images for all platforms that we can for qemu. It is
missing aarch64, and we put the image on s2 instead of
s1 and mkimg can't mark s2 as active, so there's some
issues. Oh, and I didn't do it for arm.

Take a look, kick the tires, expect problems.